V-keel: an alternative keel setup for modern flat-bottomed hulls

Discussion in 'Sailboats' started by josvos, Dec 17, 2017.

  1. josvos
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    josvos New Member

    The more I think about it, the less it make sense to me that all today’s yachts have fin keels. Can someone please comment on this? I doubt that no one ever made the below reasoning. Please tell me where I go wrong.

    The fin keel has evolved from the S-hull, where it did make sense. But today yachts are wide with relatively flat bottoms. With this setup, the keel attaches to the boat at the point where it has the lowest “shape-wise stiffness”: in the middle of a plane surface.
    Also, the keel (and the attachment of the keel to the boat) is loaded with momentum and thus high internal stresses, a surface that you want to be as thin as possible.

    Wouldn’t it make more sense if we would change to a V-keel? In this setup, the keel would not have one central foil, but two foils to the chines of the boat. (see also attachment)

    It would have several advantages:
    1. Structural advantage
    On a standard keel, the foil is loaded by the counter load with momentum, so it needs to be really strong. Internally the boat must be significantly reinforced to transfer the momentum.
    The foils in the V-keel are only loaded with tensile and compressive loads, so their sectional dimensions can be far smaller. The V-keel attaches to the hull at the chines, an inherently stiff point of the hull.
    The equivalent of the canting fin keel would be a V-keel with foils adjustable in length. Again no momentum transfer would be required only extension or contraction of the foils (e.g. in a telescopic way)
    2. hydrodynamic advantage
    If you would create foils with positive lift on the regions of the foils closest to the chines, you could create a hydrodynamic advantage. If one would extend the foils outside the beam of the boat and create lift there, a DSS-like system would be obtained.
    In summary, advantages of different today’s systems could be obtained with less appendages.

    Different setups could be worked out for cruiser and racers. For more explanation, please look at the attachment.

    Elaboration of some of the technical parts could be challenging, for example the point where the extendable foils meet the boat or the extension system itself in case of a movable ballast. But the equivalent systems for a fin keel must have been at least as challenging when the development of the canting keel was started.

    I know the idea is not 100% new. It might remind you of early trials with dinghies with lifting foils, and in a way it is not very different, only with a keel now. How come this has never been picked up again and elaborated?

    Attached Files:

  2. gggGuest
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    gggGuest ...

    Structurally, yes, you're right it has some major advantages. Nothing like a triangle for adding rigidity.

    However really this can be looked at as a variation of the twin or bilge keel.

    So far the problem with all variations on the twin keel principle is that the hydrodynamic side is not great. It could be argued that compared to a more conventional twin keel installation this takes away some of the advantages - easy sitting on the bottom for instance, adds some new ones - the interactions between the two foils at the keel would be very complicated. The lift concept is very high profile at the moment but I'm not sure how applicable it will end up being beyond very high powered and light weight racing craft.

    It might be an interesting exercise if you were to compare your design's hydrodynamic advantages and disadvantages with those of two independent single keels and see where you end up.

    I don't doubt you could build a usable boat with a keel like that, but the big question is, bearing in mind that every concept has pros and cons, what are the pros and cons of your configuration against all the alternatives? Its very easy to get enormously enthused by the advantages of a new concept, but is also very valuable to study the disadvantages. In what circumstances are all the disadvantages minimised, and how often will that occur? In what circumstances is your concept better than *every* alternative?
    Last edited: Dec 17, 2017
    kapnD and jorgepease like this.
  3. rwatson
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    rwatson Senior Member

    Well, your are wrong. This concept is very very old. I cant be bothered looking up the reference, but early foiling configurations tried all the variants.

    The obvious problem has been pointed out - hydrodynamic flow interference, which is why bilge keels aren't popular for competitive racing.
  4. josvos
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    josvos New Member

    Hi gggGuest,

    Thanks for your response.
    The concept indeed comparable to the bilge keel, in the way that these also attach to the chine. But bilge keels are usually short and thick. Here we could make thinner foils than a fin keel.

    My assessment of the disadvantages is rather the hydrodynamic situation around the place where the foils attach to the chine. Since the foil does not attach to the hull in a perpendicular way, the disturbed area will be greater than with other foils. Around the bulb, I expect the interference to be limited, since the foils are perpendicular here.

    I agree it is is tempting to be too enthusiastic about a (more or less) new concept. Therefore, any comments are most welcome.
  5. sharpii2
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    sharpii2 Senior Member

    Another disadvantage of this idea is that the lee ward foil would have a definite upward lift component, as the boat heels. This would add to the capsize moment, as it is likely to be windward of the Lateral Center of Buoyancy (LCB) .

    The main reason bilge keels are shallow and stubby is so the boat can dry out between tides, without flopping over. They also tend to help the boat track better, as they resist turning.
  6. pogo
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    pogo ingenious dilletante

  7. CT249
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    CT249 Senior Member

    I can't read the bottom article, but looking at it there appears to be an obvious question - does the twin keel boat go faster than a comparable single fin? It appears from the boxes showing specifications that the designer was comparing his twin keel design to the much heavier Soling, and also just assuming that his boat was dramatically faster than a Dyas.

    If his twin keeler was dramatically faster shouldn't there be race times and handicaps to prove it? It appears that the Fighter, which uses such a keel, is not particularly fast. The yardstick is very close to that of the 1940s designed Flying 15 keelboat, which is shorter and has a much smaller rig. The Fighter is also much slower than the Dyas and Soling.
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